This invention relates to star polymers of hydrogenated isoprene and styrene, and to oil compositions containing the star polymers.
The viscosity of lubricating oils varies with temperature. In general, oils are identified by a viscosity index which is a function of the oil viscosity at a given lower temperature and a given higher temperature. The given lower temperature and the given higher temperature have varied over the years but are fixed at any given time in an ASTM test procedure (ASTM D2270). Currently, the lower temperature specified in the test is 40.degree. C. and the higher temperature specified in the test is 100.degree. C.
Numerous methods have been proposed for improving the rheological properties of lubricating oil compositions. Generally, these methods involve the use of a polymeric additive which cannot be handled as a solid. More recently, it has been discovered that certain asymmetric star-shaped polymers are both solid polymers and effective VI improvers for lubricating oil compositions as disclosed in U.S. Pat. No. 4,849,481. The patent describes asymmetric star polymers of conjugated diolefin blocks, such as polyisoprene, and monoalkenyl aromatic blocks, such as polystyrene. The star polymers contain a substanial amount of the monoalkenyl aromatic blocks and produce oil compositions having reduced high temperature, high shear rate (HTHSR) viscosities. The asymmetric star polymers have sufficient amounts of the monoalkenyl aromatic blocks to be cyclone finishable as a polymer crumb.
Although reduced HTHSR viscosities likely indicate good fuel efficiency, increased HTHSR viscosities are needed for high performance engines which operate at higher temperatures and rpms, or for engines having marginally designed journal bearings. Minimum HTHSR viscosities have been proposed as an industry standard. Star polymers having a sufficiently low amount of the monoalkenyl aromatic blocks to have a high HTHSR viscosity were expected to not have a sufficient amount of the monoalkenyl aromatic blocks to be cyclone finishable as a polymer crumb or were expected to cause gelation.
Process oils which detract from VI improver properties often must be added to non-cyclone finishing polymers in order to finish the polymers by falling strand methods. For this reason, star polymers having low amounts of the monoalkenyl aromatic blocks were not specifically described or claimed in U.S. Pat. No. 4,849,481. FIG. 1 of the patent does suggest that oil compositions such as multigrade SAE 10W-40 (graded according to the SAE J300 standard of the SAE Engine Oil Viscosity Classification System) can be produced with the asymmetric star polymers having a low monoalkenyl aromatic block content. However, the patent does not suggest that the star polymers having low amounts of the monoalkenyl aromatic blocks would be both cyclone finishable and useful in preparing oil compositions having higher HTHSR viscosities.